1. Field of the Invention
This invention relates to air conditioning systems, heat pump systems, and, integrated heat pump systems that provide water heating. The invention is more particularly directed to a compressor current control for such systems which have a variable speed compressor.
2. Description of the Prior Art
Integrated heat pumps are often employed to provide heating or cooling, as needed, to a residential or commercial comfort zone, i.e., the interior of a residence, office complex, hospital or the like. Integrated heat pumps are also employed to heat water. A heat pump for air conditioning, comfort zone heating, and water heating is described in U.S. Pat. No. 4,766,734 entitled Heat Pump Systems with Hot Water Defrost to Kevin F. Dudley. Systems of this type typically have several modes of operations, such as air conditioning alone, comfort zone space heating alone, air conditioning with water heating, and comfort zone space heating with water heating.
Such systems are now commonly equipped with variable speed compressors in which the speed of the motor driving the compressor may be varied by speed command signals from a controller, which, in turn, is responding to sensed ambient and system conditions. In general, the motor speed is increased for operation under high load conditions and decreased for operation under low load conditions.
It has been found that when the load on the variable speed compressor is increased at a given operating speed, the current to the compressor increases until the current limit of the variable speed drive is reached. Once the current limit of the variable speed drive is reached, however, if the load upon the compressor is increased further, the speed of the compressor will decrease due to the current being limited. This situation will eventually lead to shut down of the compressor as the speed drops to zero.